Micro-LED Movement
Micro-LEDs are a display technology that has the potential to revolutionize the display industry, realizing the goals of ultra-high resolution displays that have the ability to create images that mimic what the human eye can see. With a 120° x 120° field of view, the human eye would need the equivalent of 576 megapixels[1] to fully capture the details of a scene, and with 8.29 megapixels in a 4K display there is lots of room for improvement with such a display having pixels 0.375mm apart while the human eye equivalent would have pixels 0.0486mm apart for the same 65” screen. Of course these are theoretical numbers and the human eye does not take in an entire scene all at once, but the idea that higher resolutions lead to more realistic images is one that has driven the display industry since its inception.
Each display technology that has come along since Philo Farnsworth created the first television prototype in the 1920’s has strived toward creating images that give us the impression we are looking at a real scene and not a flat image on a display, with much of that technology trying to squeeze more pixels onto that screen and that means making pixels smaller and squeezing more of them together and that has caused display engineers to look for methods to make pixels and sub-pixels progressively smaller, but along with smaller ‘dots’ comes more precise tools and that leads to both technical and financial complications and solving those issues can take years and billions of dollars and once a solution is found and infrastructure must be built to support the technology.
Micro-LEDs are essential the same as the LEDs seen in lighting applications only much smaller, and by much smaller we mean very much smaller, with sizes down to 2um, and while growing such almost microscopic semiconductors on a silicon wafer is certainly possible, it also creates a number of problems, one of which is transferring those tiny LEDs to a display substrate, which we have noted on a number of occasions, but there are issues that are equally onerous, some of which are due to the fact that each pixel in a micro-LED display is made up of three sub-pixels, one each of red, green, and blue. Green and blue LEDs are based on InGaN (Indium Gallium Nitride), while red LEDs are based on InGaP (Indium Gallium Phosphide), which means, in theory, that blue and green LEDs can be grown on the same silicon wafers, but red LEDs must be grown on a separate wafer, increasing the complexity of transferring the LEDs to a display substrate, and to add insult to injury, as red LEDs get smaller they become less efficient and do not match the characteristics of blue and green LEDs, making the creation of a balanced pixel more complex.
One workaround that has become a focus for Micro-LED display engineers is to use only blue LEDs in each pixel and cover the pixels with a sheet containing patterns of red and green quantum dots. The dots convert the blue light to red and green, while letting the blue light pass through, similar to the way WOLED displays use a color filter to create color by passing white OLED light through a color filter, essentially a sheet of red, green, an blue dots. That said, color filters remove much of the light, while quantum dots shift colors with relatively little loss. This helps to maintain a bright display and lessens the need for multiple OLED stacks or other enhancements that reduce the lifetime of OLED materials.
Again, the practicality of Micro-LEDs is not perfect and while quantum dots are a more efficient way to generate color, they are not perfect and coating a Micro-LED with a thick film of quantum dots leads to a mis-match of the quantum dots to the blue Micro-LEDs, which reduces the efficiency of the conversion. Figure 2 illustrates how the mis-alignment of the red and green quantum dots in the QD film can reduce the conversion efficiency of the converted light. However a small company in Branfod, CT, Saphlux (pvt), a spin-off of Yale University, has come up with a solution that seems to have attracted the attention of a number of US and Chinese venture funds and Shenzhen Leyard Opto-Electronics (300296.CH) among the top Chinese A/V technology companies, who has just announced it has begun mass production of the NPQD R1 Micro-LED chip that was jointly developed by both parties.
Saphlux has developed a process that by electro-chemically etching they create what they call ‘micro-pores’ in GaN doped with Silicon, which allows them to reduces the quantum dot thickness layer by 50% and increases the light-conversion efficiency from 20% to 80% and improving wavelength (color) uniformity. Once the micro-pores are created they pattern red and green quantum dots (leaving spaces for the blue to shine through), which fill the pores. Due to the internal light scattering inside the nano-pores, the conversion efficiency is greatly improved, with the process using standard photolithography or inkjet printing, keeping costs low and the improvement in wavelength uniformity reduces the issue of ‘binning’ where each LED must be categorized and possibly skipped during transfer in order to create a display with uniform color and brightness.
All in, we see progress across a number of fronts relative to the practical commercialization of micro-LEDs, and while price comparisons to existing display modalities are still years away, over the last two years considerable movement in micro-LED has been seen, with quantum dots a way to both lessen the difficulty of producing RGB micro-LEDs and the complexity of micro-LED transfer. Some of this push toward commercialization comes from the AR/VR space where there is a very distinct need for high resolution displays, but the attraction of applications in the more traditional CE space, encompassing TV and IT products including mobile devices, and the high unit volumes they generate seems to have lit a fire under Micro-LED R&D and process engineering that has moved things along more quickly than we had imagined. We are not at a true commercial level yet, but the number of roadblocks is declining and that will encourage more focus on the Micro-LED space going forward.
[1] (Clark, Notes on the Resolution and Other Details of the Human Eye, 2005)
Each display technology that has come along since Philo Farnsworth created the first television prototype in the 1920’s has strived toward creating images that give us the impression we are looking at a real scene and not a flat image on a display, with much of that technology trying to squeeze more pixels onto that screen and that means making pixels smaller and squeezing more of them together and that has caused display engineers to look for methods to make pixels and sub-pixels progressively smaller, but along with smaller ‘dots’ comes more precise tools and that leads to both technical and financial complications and solving those issues can take years and billions of dollars and once a solution is found and infrastructure must be built to support the technology.
Micro-LEDs are essential the same as the LEDs seen in lighting applications only much smaller, and by much smaller we mean very much smaller, with sizes down to 2um, and while growing such almost microscopic semiconductors on a silicon wafer is certainly possible, it also creates a number of problems, one of which is transferring those tiny LEDs to a display substrate, which we have noted on a number of occasions, but there are issues that are equally onerous, some of which are due to the fact that each pixel in a micro-LED display is made up of three sub-pixels, one each of red, green, and blue. Green and blue LEDs are based on InGaN (Indium Gallium Nitride), while red LEDs are based on InGaP (Indium Gallium Phosphide), which means, in theory, that blue and green LEDs can be grown on the same silicon wafers, but red LEDs must be grown on a separate wafer, increasing the complexity of transferring the LEDs to a display substrate, and to add insult to injury, as red LEDs get smaller they become less efficient and do not match the characteristics of blue and green LEDs, making the creation of a balanced pixel more complex.
One workaround that has become a focus for Micro-LED display engineers is to use only blue LEDs in each pixel and cover the pixels with a sheet containing patterns of red and green quantum dots. The dots convert the blue light to red and green, while letting the blue light pass through, similar to the way WOLED displays use a color filter to create color by passing white OLED light through a color filter, essentially a sheet of red, green, an blue dots. That said, color filters remove much of the light, while quantum dots shift colors with relatively little loss. This helps to maintain a bright display and lessens the need for multiple OLED stacks or other enhancements that reduce the lifetime of OLED materials.
Again, the practicality of Micro-LEDs is not perfect and while quantum dots are a more efficient way to generate color, they are not perfect and coating a Micro-LED with a thick film of quantum dots leads to a mis-match of the quantum dots to the blue Micro-LEDs, which reduces the efficiency of the conversion. Figure 2 illustrates how the mis-alignment of the red and green quantum dots in the QD film can reduce the conversion efficiency of the converted light. However a small company in Branfod, CT, Saphlux (pvt), a spin-off of Yale University, has come up with a solution that seems to have attracted the attention of a number of US and Chinese venture funds and Shenzhen Leyard Opto-Electronics (300296.CH) among the top Chinese A/V technology companies, who has just announced it has begun mass production of the NPQD R1 Micro-LED chip that was jointly developed by both parties.
Saphlux has developed a process that by electro-chemically etching they create what they call ‘micro-pores’ in GaN doped with Silicon, which allows them to reduces the quantum dot thickness layer by 50% and increases the light-conversion efficiency from 20% to 80% and improving wavelength (color) uniformity. Once the micro-pores are created they pattern red and green quantum dots (leaving spaces for the blue to shine through), which fill the pores. Due to the internal light scattering inside the nano-pores, the conversion efficiency is greatly improved, with the process using standard photolithography or inkjet printing, keeping costs low and the improvement in wavelength uniformity reduces the issue of ‘binning’ where each LED must be categorized and possibly skipped during transfer in order to create a display with uniform color and brightness.
All in, we see progress across a number of fronts relative to the practical commercialization of micro-LEDs, and while price comparisons to existing display modalities are still years away, over the last two years considerable movement in micro-LED has been seen, with quantum dots a way to both lessen the difficulty of producing RGB micro-LEDs and the complexity of micro-LED transfer. Some of this push toward commercialization comes from the AR/VR space where there is a very distinct need for high resolution displays, but the attraction of applications in the more traditional CE space, encompassing TV and IT products including mobile devices, and the high unit volumes they generate seems to have lit a fire under Micro-LED R&D and process engineering that has moved things along more quickly than we had imagined. We are not at a true commercial level yet, but the number of roadblocks is declining and that will encourage more focus on the Micro-LED space going forward.
[1] (Clark, Notes on the Resolution and Other Details of the Human Eye, 2005)
LED Size Comparisons - Source: Evandesigns.com
Micro-LED & QD Film - Source: SCMR LLC
Top view & Cross section images of anao-porous GaN at different etch voltages - Source: Saphlux
Saphlux etch and patterning – Source: Saphlux
Saphlux RGB patterned micro-LED array - 36um x 36um - Source: Saphlux
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